1. Field of the Invention
The present invention relates to a spin-transfer torque (STT) magnetic random access memory (MRAM), and, more particularly, to an STTMRAM element having magnetic moments oriented perpendicular to the plane of the substrate, and having lower programming current density while maintaining higher thermal stability.
2. Description of the Prior Art
Magnetic random access memory (MRAM) and resistive RAM (RRAM), which are a type of non-volatile memory, have gained great notoriety within recent years; however, efforts to improve their practicality of manufacturing and operation are well under way. These types of memory, which switch between a parallel and an anti-parallel states through the application of spin polarized current during programming, is also being investigated.
One type of MRAM is spin-transfer torque magnetic random access memory (STTMRAM). While STTMRAM are expected to be a universal memory solution, various hurdles stand in the way. The programming current of STTMRAM is still very high as main memory, therefore, the cell size is too large thereby preventing its adoption in consumer devices. Scaling STTMRAM below 65 nm is ideal; however reducing the programming current and shrinking the cell size introduces a variety of issues, such as thermal instability.
STTMRAM has significant advantages over magnetic-field-switched (toggle) MRAM, which has been recently commercialized. The main hurdles associated with field switched MRAM are its more complex cell architecture, which utilizes bypass line and remote write lines. Additional hurdles include its high write current (currently in the order of milliamps (mA)) and poor scalability, which cannot scale beyond 65 nano meters (nm). The fields and the currents required to write the bits increase rapidly as the size of the MTJ elements shrink. STT writing technology, by directly passing a current through the MTJ, overcomes these hurdles with much lower switching current (in the order of microamps (uA)), simpler cell architecture which results in a smaller cell size (for single-bit cells) and reduced manufacturing cost, and more importantly, improved scalability.
One of the challenges for implementing STT, is that during writing mode, such memory used in high-density and high-speed memory applications, require substantial reduction of the intrinsic current density to switch the magnetization of the free layer while maintaining high thermal stability, which is required for long-term data retention. Minimal switching (write) current is required mainly for reducing the size of the select transistor of the memory cell, which is typically coupled, in series, with MTJ to achieve the highest possible memory density. The channel width (in unit of F) of the transistor is proportional to the write current for a given transistor current drivability (uA/um). Minimal channel width or the width of MTJ element is required for achieving a reduced STTMRAM cell size. Second, smaller voltage across MTJ decreases the probability of tunneling barrier degradation and breakdown, ensuring write endurance of the device. This is particularly important for STTMRAM, because both sense and write currents are driven through MTJ cells.
One of the efficient way to reduce the programming current in STTMRAM is to use a MTJ with perpendicular anisotropy.
Incorporation of MgO-based MTJ with conventional perpendicular anisotropy material(s), such as FePt, into STTMRAM with such MTJ designs cause high damping constant, high magnetic anisotropy leading to undesirably high switching current density. Furthermore, during manufacturing, conventional higher ordering transformation temperature required for forming L10 order structure could undesirably degrades or even obliterates tunneling magneto-resistance (TMR) performance.
What is needed is a STTMRAM element having a MTJ with perpendicular magnetic anisotropy material(s) with an optimal combination of saturation magnetization (Ms) and anisotropy constant (Ku) to lower the damping constant and the magnetic anisotropy of the MTJ yielding a lower switching current density associated with the MTJ while maintaining high thermal stability and high TMR performance.